The first machine on site looks like it is making a mess. It is actually setting every level on the job. Everything built afterwards is measured from what the dig leaves behind.
Clearance and excavation is the whole of the beginning. Scrub, stumps, old slabs, a forgotten redundant tank (a fuel oil tank is a specialist decommissioning job in its own right). All of it goes. Then the topsoil comes off, the building is set out, and the ground is cut to the levels the engineer asked for. An inaccurate dig cannot be papered over by the trades that follow. Only paid for.
Why nobody builds on topsoil
Topsoil is alive. Roots, worms, organic matter quietly rotting. What makes it good in a garden makes it useless under a building. Put a load on it and it compresses. As the organic content breaks down, the volume drops away. Nothing structural goes on topsoil. Not a foundation, not a slab, not a drive, not even the sub-base under a path.
So it comes off. How deep depends on the site. A few hundred millimetres on a well-drained plot, considerably more where the ground is soft or made-up. The instruction is never “dig 300mm”. It is “keep going until what is underneath is undisturbed natural ground”.
WORTH KNOWING
Organic material is not the only thing that has to come out. Made-up ground, buried hardcore, a filled-in pond, an old brick footing across the plot. Soft spot rather than bearing ground, dug out and replaced with compacted engineered fill. It is the commonest reason a dig ends up deeper than the drawing said.
The order the work happens in
The sequence rarely changes, whatever the size of the job.
Find out what is buried
Before anything with teeth on it moves, the services get located. Plans from the utility companies, then the ground scanned with a cable avoidance tool. HSE guidance HSG47 is blunt: drawings alone are not enough. Private runs are the hazard. A stable block supply, an abandoned oil line, a land drain on nobody’s records.
Clear and strip
Scrub and self-set trees down, stumps and root plates grubbed out rather than cut off at the collar, hardstanding broken up, outbuilding bases out. Then the organic layer comes off in a controlled strip. Good topsoil is worth stockpiling clear of the works. Buying it back in at landscaping stage is a poor trade.
Set out and cut to formation
Setting out puts the building exactly where the drawings say, tied to a fixed datum so the level can be found again weeks later. Then the site is cut to formation, the trimmed, cleaned-off surface everything above it is built from. Formation is not an approximation of a level. It is the level.
Trench and dig deep
Foundation trenches, drain runs and service trenches, dug to the drawing and kept clean and dry until inspected. Where there is a basement or lower-ground floor, this is where most of the job sits. And most of the lorries.
Bulk dig, reduced level and trenching
- A reduced-level dig takes the whole platform down to one level, cutting a slope into a workable terrace, or getting below the soft ground.
- A bulk dig is volume work: a large hole taken out in one operation, priced by the cubic metre. Basements, pools, plant rooms, big level changes.
- Trenching is narrow, deep and dearer per cubic metre. The machine is confined to a line and the spoil is handled twice.
Deep excavation for a basement or lower-ground floor is the same trade taken further, though the dig is only half of it. Something has to hold the sides open while work goes on inside, and that temporary support is designed and installed as its own specialist package. The design is somebody else’s. A structural engineer designs the hole and what holds the ground back, and a waterproofing specialist keeps the finished rooms dry. Ask who those two are before the first bucket. Our guide to basements and big digs walks the sequence through.
You are not paying somebody to dig a hole. You are paying them to know when to stop.
Muck-away, and why it is rarely padding
Everything that comes out has to go somewhere, and this is the line clients mistake for fat. Start with bulking. Soil in the ground is packed tight. Dug, it breaks up and takes more room. Stiff clay commonly gains twenty to thirty per cent in volume. A hundred cubic metres in the ground is comfortably more than a hundred on a lorry.
Then the paperwork. Soil leaving a site is controlled waste: carried by someone registered with the Environment Agency, described accurately in writing, taken somewhere licensed, with transfer notes kept for two years. The duty of care sits with whoever produced it, not only with the driver. If a muck-away figure looks cheap, ask where the material is going.
Then arithmetic. A big eight-wheeler carries something in the region of sixteen to twenty tonnes, and less loose volume than people picture. On a narrow lane in Gerrards Cross or Beaconsfield, or anywhere across the villages we cover, a dig runs at the speed of the lorries, not the machine.
Trees, access and the size of the machine
This part of the county is heavily treed, and the trees often carry more legal protection than the building does. A tree under a Preservation Order needs the council’s consent before anyone touches it. In a conservation area, even an unprotected tree above roughly 75mm trunk diameter normally needs six weeks’ written notice to the local planning authority. Check with yours before anything is felled. Getting it wrong is a criminal matter, not a fee.
Trees that stay bring a root protection area with them: a circle around the trunk that machines keep out of. As a rule of thumb the radius is twelve times the trunk diameter, capped for the largest trees, though your arboriculturist confirms it. Inside that circle, work goes on by hand. Roots do not grow back.
Access decides the rest. A plot a thirteen-tonne excavator can reach is a different job from one worked with a three-tonne machine and a dumper through a side gate. Overhead cables, a weight limit on a private road, a gateway too narrow for a wagon: none of it can be judged from a photograph, which is why the site visit comes before the figure.
ASK ANY CONTRACTOR
- What depth of strip has this assumed, and what is the rate if good ground is lower?
- Has anyone dug trial holes, or is this priced off drawings alone?
- How many loads of muck-away are allowed for, and who pays for the rest?
- Which trees are staying, and what protects them while machines work?
What the ground does to the programme
Ground will not negotiate. Around here it is a patchwork. Chalk, gravel, clay-with-flints and London Clay within a few streets of each other. Chalk digs easily, then softens under rain and tracked plant. Clay stands up in a dry August and slumps in November. Gravel runs, faster if the water table sits in it. There is more in what actually happens below your home.
Two things follow. Support is a design question, not a depth question. The notion that a trench under 1.2 metres needs none belongs to regulations revoked in the 1990s. The duty is to prevent collapse at any depth. And the bottom of the hole is a decision point: building control looks in before concrete goes near it, and if there are roots or soft ground down there, the dig goes deeper that day. Our guide to foundations sets out who decides the depth.
The best compliment a dig ever gets is that nobody mentions it again. Get it right and the foundations, the drainage and the drive all land where they should. Get it wrong and every trade after you works around it for the rest of the build.
Access and protected trees change this work village by village. There is more on the ground in each of these.
Planning something on your own plot?
Request a Quote